LED lighting system with accurate current control

ABSTRACT

A light emitting diode (LED) lighting system and method are disclosed. The LED lighting system and method include an LED controller to accurately control a current in an LED system. The LED controller includes components to calculate, based on the current and an active time period of an LED current time period, an actual charge amount delivered to the LED system wherein the LED current time period is duty cycle modulated at a rate of greater than fifty (50) Hz and to utilize the actual charge amount to modify and provide a desired target charge amount to be delivered during a future active time period of the LED current time period. The LED system and method further involve components to compare the actual charge amount to a desired charge amount for the active time period and compensate for a difference between the actual charge amount and the desired charge amount during the future active time period.

CROSS REFERENCE TO RELATED APPLICATIONS

U.S. Provisional Application No. 60/909,458, entitled “Ballast for LightEmitting Diode Light Sources,” inventor John L. Melanson, and filed onApr. 1, 2007 describes exemplary methods and systems and is incorporatedby reference in its entirety. Referred to herein as “Melanson I”.

U.S. patent application Ser. No. 12/047,249, entitled “Ballast for LightEmitting Diode Light Sources,” inventor John L. Melanson, and filed onMar. 12, 2008 describes exemplary methods and systems and isincorporated by reference in its entirety. Referred to herein as“Melanson II”.

U.S. patent application Ser. No. 12/047,269, entitled “Lighting Systemwith Power Factor Correction Control Data Determined from a PhaseModulated Signal,” inventor John L. Melanson, and filed on Mar. 12, 2008describes exemplary methods and systems and is incorporated by referencein its entirety. Referred to herein as “Melanson III”.

U.S. patent application Ser. No. 11/695,024, entitled “Lighting Systemwith Lighting Dimmer Output Mapping,” inventors John L. Melanson andJohn Paulos, and filed on Apr. 1, 2007 describes exemplary methods andsystems and is incorporated by reference in its entirely. Referred toherein as “Melanson IV”.

U.S. patent application Ser. No. 11/864,366, entitled “Time-BasedControl of a System having Integration Response,” inventor John L.Melanson, and filed on Sep. 28, 2007 describes exemplary methods andsystems and is incorporated by reference in its entirety. Referred toherein as Melanson V.

U.S. patent application Ser. No. 11/967,269, entitled “Power ControlSystem Using a Nonlinear Delta-Sigma Modulator with Nonlinear PowerConversion Process Modeling,” inventor John L. Melanson, and filed onDec. 31, 2007 describes exemplary methods and systems and isincorporated by reference in its entirety. Referred to herein asMelanson VI.

U.S. patent application Ser. No. 11/967,271, entitled “Power FactorCorrection Controller with Feedback Reduction,” inventor John L.Melanson, and filed on Dec. 31, 2007 describes exemplary methods andsystems and is incorporated by reference in its entirety. Referred toherein as Melanson VII.

U.S. patent application Ser. No. 11/967,273, entitled “System and Methodwith Inductor Flyback Detection Using Switch Gate Charge CharacteristicDetection,” inventor John L. Melanson, and filed on Dec. 31, 2007describes exemplary methods and systems and is incorporated by referencein its entirety. Referred to herein as Melanson VIII.

U.S. patent application Ser. No. 11/967,275, entitled “ProgrammablePower Control System,” inventor John L. Melanson, and filed on Dec. 31,2007 describes exemplary methods and systems and is incorporated byreference in its entirety. Referred to herein as Melanson IX.

U.S. patent application Ser. No. 11/967,272, entitled “Power FactorCorrection Controller With Switch Node Feedback”, inventor John L.Melanson, and filed on Dec. 31, 2007 describes exemplary methods andsystems and is incorporated by reference in its entirety. Referred toherein as Melanson X.

U.S. patent application Ser. No. 12/058,971, entitled “LED LightingSystem with a Multiple Mode Current Control Dimming Strategy”, inventorJohn L. Melanson, and filed on Mar. 31, 2008 describes exemplary methodsand systems and is incorporated by reference in its entirety. Referredto herein as Melanson XI.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates in general to the field of electronics andlighting, and, more specifically, to a light emitting diode (LED) systemand method with accurate current control.

Description of the Related Art

Commercially practical incandescent light bulbs have been available forover 100 years. However, other light sources show promise ascommercially viable alternatives to the incandescent light bulb. LEDsare becoming particularly attractive as main stream light sources inpart because of energy savings through high efficiency light output andenvironmental incentives such as the reduction of mercury.

LEDs are semiconductor devices and are driven by direct current. Thelumen output intensity (i.e. brightness) of the LED approximately variesin direct proportion to the current flowing through the LED. Thus,increasing current supplied to an LED increases the intensity of the LEDand decreasing current supplied to the LED dims the LED, i.e. decreasesthe brightness of the LED. Current can be modified by either directlyreducing the direct current level to the white LEDs or by reducing theaverage current through duty cycle modulation.

Dimming a light source saves energy when operating a light source andalso allows a user to adjust the intensity of the light source to adesired level. Many facilities, such as homes and buildings, includelight source dimming circuits (referred to herein as “dimmers”).

FIG. 1 depicts an LED lighting system 100 that supplies power to lightemitting diodes (LEDs) 102 and dims the LEDs 102 in accordance with adimming level indicated by the phase modulated signal V_(Φ). The voltagesource 104 supplies an alternating current (AC) input voltage V_(IN).Full, diode bridge rectifier 108 rectifies the input voltage V_(IN). Themains voltage source 104 is, for example, a public utility, and theinput voltage V_(DIM) is, for example, a 60 Hz/120 V rectified voltagein the United States of America or a 50 Hz/230 V rectified voltage inEurope. The dimmer 106 is, for example, a phase cut dimmer thatgenerates phase delays in the rectified input voltage V_(IN) to generatephase modulated signal V_(Φ). The phase delays indicate dimming levels.Generally, as the phase delays increase, the dimming level decreases,i.e. as the phase delays increase, the dimming level indicates a lowerbrightness level for LEDs 102. The Background sections of Melanson I,Melanson II, and Melanson III, describe examples of dimmer 106.

Switching power supply 110 utilizes switching power converter technologyto convert the phase modulated signal V_(Φ) into an output voltageV_(OUT). The output voltage V_(OUT) is sufficient to bias the LEDs 102.Switching power supply 110 also supplies an LED current i_(LED) toilluminate the LEDs 102.

Current controller 112 controls active and average values of LED currenti_(LED) by controlling the conductivity of n-channel field effecttransistor (FET) Q1. Current controller 112 generates a gale controlsignal C_(G0) to charge and discharge a gate of FET Q1. The controlsignal C_(G0) has two relevant frequencies, an active frequency and aduty cycle modulated frequency. During an active period of LED currenti_(LED), the control signal C_(G0) has an active frequency in the rangeof, for example, 20 kHz to 500 kHz. As described subsequently in moredetail, the duty cycle modulated frequency is less than the activefrequency. The active period of LED current i_(LED) is the period oftime when the average value of LED current i_(LED) equals current valuei_(FULL). The time period for this average is, for example, one or a few(such as 3-5) periods of the active frequency.

When the control signal C_(G0) is a logical “one”, FET Q1 conducts, i.e.is “ON”, and when the control signal C_(G0) is a logical “zero”, FET Q1is nonconductive, i.e. is “OFF”. When the FET Q1 is “ON”, diode D1 isreversed bias and, LED current i_(LED) flows through the LEDs 102 andcharges inductor L₁. When FET Q1 is “OFF”, the voltage across inductorL₁ changes polarity, and diode D₁ creates a current path for the LEDcurrent i_(LED). The inductor L₁ is chosen so as to store enough energyto maintain an approximately constant active value of LED currenti_(LED) when MOSFET Q1 is “OFF”. Capacitor C1 helps “smooth” LED currenti_(LED). As subsequently explained in more detail, the active value ofthe LED current i_(LED) is the average LED current i_(LED) when thecurrent control system 112 is active, i.e. during the active period ofLED current i_(LED). The LED current i_(LED) includes a ripple 201 dueto, for example, the charging and discharging of inductor L1. Thefrequency of the ripple 201 is the active frequency. It is desirable,for LED efficiency, to keep the LED current relatively constant, toreduce heating effects.

FIG. 2 depicts a graphical representation 200 of the LED current i_(LED)for various dimming levels indicated by the phase modulated signalV_(Φ). Referring to FIGS. 1 and 2, when the phase modulated signal V_(Φ)indicates a full dimming level, i.e. full brightness for LEDs 102,current controller 112 controls the LED current i_(LED) so that theactive value of LED current i_(LED) is continuous and constant over timeand equals i_(FULL), as indicated by LED current i_(LED) waveform 202.“i_(FULL)” represents the active value of LED current i_(LED) thatcauses the LEDs 102 to illuminate at full brightness.

The current controller 112 uses feedback information from feedbacksignal LEDi_(sense) to sense the active value of LED current i_(LED).The feedback signal LEDi_(sense) represents a voltage V_(fb) acrosssense resistor R_(SENSE). The voltage V_(fb) represents LED currenti_(LED) when FET Q1 is ON. Thus, from the feedback signal LEDi_(sense),the current controller 112 obtains the value of LED current i_(LED) andcan adjust the duty cycle of control signal C_(G0) _(_) _(FULL) tomaintain the active value of LED current i_(LED) at the full activevalue i_(FULL) during the active period of LED current i_(LED). Assubsequently explained in more detail, the control signal C_(G0) _(_)_(FULL) is also duty cycle modulated at the duly cycle modulationfrequency in response to dimming levels indicated by phase modulatedsignal V_(Φ) to generate control signal C_(G0).

To determine the dimming level indicated by phase modulated signalV_(Φ), comparator 114 compares the phase modulated signal V_(Φ) with aphase delay detection reference signal V_(DET). The value of phase delaydetection reference signal V_(DET) is set to detect an edge of any phasedelays in the phase modulated signal V_(Φ). Generally, the edge of anyphase delays during each cycle of phase modulated signal V_(Φ) resultsin a voltage increase in phase modulated signal V_(Φ). Thus, generally,the value of phase delay detection reference signal V_(DET) is set lowenough, so that the output of comparator 114 changes from a logical 0 toa logical 1 when a rising edge associated with an end to a phase delayis detected and changes to a logical 0 if a phase delay is detectedduring a cycle of phase modulated signal V_(Φ).

Comparator 114 generates a duty cycle modulated enable signal EN at theduty cycle modulation frequency. The duty cycle of enable signal ENcorresponds to the dimming level indicated by phase modulated signalV_(Φ). The current controller 112 responds to the enable signal EN byduty cycle modulating the control signal C_(G0) so that the averagevalue, i_(LED) _(_) _(AVG), of LED current i_(LED) varies in accordancewith dimming levels indicated by the phase modulated signal V_(Φ).Modulator 116 represents a logical representation of utilizing theenable signal EN to generate a duty cycle modulated control signalC_(G0). The enable signal EN represents one input signal to AND gate118, and control signal C_(G0) _(_) _(FULL) represents another inputsignal to AND gate 118. The AND gate 118 is exemplary. In typicalapplications, the function of the AND gate 118 is integrated into thelogic of the controller 112. Control signal C_(G0) _(_) _(FULL)corresponds to control signal C_(G0) during the active period of LEDcurrent i_(LED). When the enable signal EN is a logical 1, the controlsignal C_(G0) equals the control signal C_(G0) _(_) _(FULL). When theenable signal EN is a logical 0, the control signal C_(G0) equals 0.Thus, the control signal C_(G0) is duty cycle modulated to generate thecontrol signal C_(G0) _(_) _(FULL) and is duty cycle modulated inresponse to the phase modulated signal V_(Φ).

For example, referring to LED current i_(LED) waveform 204, when thephase modulated signal V_(Φ) indicates a ¾ dimming level, the duty cycleof enable signal EN is 0.75. The enable signal EN causes the currentcontroller 112 to duty cycle modulate the control signal C_(G0) with thesame duty cycle as enable signal EN so that time period T_(ACTIVE) _(_)_(3/4)/T equals 0.75. Thus, the active period of LED current i_(LED)equals T_(ACTIVE) _(_) _(3/4) for each period T of phase modulatedsignal V_(Φ) while the phase modulated signal V_(Φ) indicates a ¾dimming level. Period T represents a duty cycle modulated period, andthe duty cycle modulated frequency equals 1/T. The average LED currenti_(LED) _(_) _(AVG) equals i_(FULL) (the active value of LED currenti_(LED)) times the duty cycle of enable signal EN. For a ¾ dimminglevel, the average LED current i_(LED) _(_) _(AVG) equals 0.75·i_(FULL).During the inactive period of LED current i_(LED), i.e. between the endof the active period T_(ACTIVE) _(_) _(3/4) and the beginning of thenext period of phase modulated signal V_(Φ), the LED current i_(LED) iszero.

Referring to LED current i_(LED) waveform 206, when the phase modulatedsignal V_(Φ) indicates a ⅛ dimming level, the duty cycle of enablesignal EN is 0.125. The enable signal EN causes the current controller112 to duty cycle modulate the control signal C_(G0) with the same dutycycle as enable signal EN so that time period T_(ACTIVE) _(_) _(1/8)/Tequals 0.125. Thus, the active period of LED current i_(LED) equalsT_(ACTIVE) _(_) _(1/8) for each period T of phase modulated signal V_(Φ)while the phase modulated signal V_(Φ) indicates a ⅛ dimming level. Theaverage LED current i_(LED) _(_) _(AVG) equals i_(FULL) times the dutycycle of enable signal EN. For a ⅛ dimming level, the average LEDcurrent i_(LED) _(_) _(AVG) equals 0.125·i_(FULL). During the inactiveperiod of LED current i_(LED), i.e. between the end of the active periodT_(ACTIVE) _(_) _(1/8) and the beginning of the next period of phasemodulated signal V_(Φ), the LED current i_(LED) is zero.

Dimmable LED systems are typically driven with a Pulse Width Modulation(PWM) controlling a constant-current source, and the PWM duty cycle ismodified to select the dimming level. The constant current source iseither linear or a switch-mode controller. For most high powered LEDapplications, such as general lighting, system efficiency is a criticalcharacteristic, and a switch-mode controller is used. The switchingfrequency f_(SW) of the controller is typically in the range of 20 kHzto 1+ MHz. Examples of switch mode controllers would be the SipexCorporation SP6652 and the National Instruments LM3407. The datasheetsfor Sipex Corporation SP6652 and National Instruments LM3407respectively dated May 25, 2007 and Jan. 18, 2008 are herebyincorporated by reference.

Referring now to FIG. 3, an exemplary plot 300 of an Enable signal EN,output voltage V_(out), and LED current I_(LED) against dimming voltagevalues is shown for the Sipex SP6652. Exemplary ramp-up 302 of an activeperiod for LED current I_(LED) is shown in plot 300. In FIG. 3, thevoltage V_(IN) is 4.2 Volts, V₀ is 3.3 Volts, I_(out) is 600 mA,R_(sense) equals 4 kohm, and L₁ equals 4.7 microH. Furthermore,referring now to FIG. 4, another exemplary plot 400 of an Enable signalEN, output voltage V_(out), and LED current I_(LED) against dimmingvoltage values is shown for the Sipex SP6652. Another exemplary ramp-up402 of an active period for LED current I_(LED) is shown in plot 400. InFIG. 4, the voltage V_(IN) is 4.2 Volts, V₀ is 1.5 Volts, I_(out) is 600mA, R_(sense) equals 4 kohm, and L₁ equals 4.7 microH. As shown in plots300 and 400, the shape of LED current I_(LED) is not controlled verywell.

Referring now to FIG. 5, an exemplary plot 500 of Enable signal V_(EN),output voltage V_(LED), and LED current I_(LED) against time is shownfor the National Semiconductor LM3407. Specifically in plot 500,enabling of dimming is shown, and exemplary current ramp-up 502 of anactive period for LED current I_(LED) is shown. Referring now to FIG. 6,an exemplary plot 600 of Enable signal V_(EN), output voltage V_(LED),and LED current I_(LED) against time is shown for the NationalSemiconductor LM3407. Specifically in plot 600, disabling of dimming isshown, and exemplary current ramp-down 602 of the same active period forLED current I_(LED) as shown in plot 500 is shown.

The switch mode controllers (e.g., Sipex SP6652 and NationalSemiconductor LM3407) have an enable input signal (e.g., Enable signalEN or V_(EN)) that is pulsed for PWM operation. Ideally, a desiredamount of charge for each active time period for LED current I_(LED) isdesired to be provided to the LED system. However, due to limitations ofquantizing charge in discrete time, providing an ideal desired amount ofcharge for an active time period for LED current I_(LED) to an LEDsystem is very hard or impossible to achieve. Such non-idealities are initself due to the nature of charge quantization (e.g., charge quantizingcycles). The inherent problem of quantizing cycles of charge is that itis limited to the exactness of the amount of charge of LED currentI_(LED) being provided to the LED system due to the fact that charge isquantized in discrete amounts based on discrete time. For example, thegeometric points in time of when LED current I_(LED) ramp-up (e.g.,slope 502) and ramp-down (e.g., slope 602) and the cycle rate at whichthe LED current I_(LED) fluctuates at an average peak current value inaccordance with the values of pulses of a control signal limit theexactness of the amount of charge being provided. Also, temperaturevariations, power supply variations, LED aging, etc. also impact theaccuracy of the amount of charge being delivered to an LED system.

Also, too slow of a PWM operation frequency (e.g., below 200 Hz) forpulsing the enable input signal can be perceived as a flicker of the LEDof a dimmable LED lighting system. Furthermore, operation below a PWMfrequency of 20 kHz for pulsing the enable input signal has thepotential to create audio tones due to acoustic behavior of magneticmaterial, which is undesirable and can lead to higher cost to amelioratethe sound path.

On the other hand, an overly fast PWM operation frequency for pulsingthe enable input signal runs into a problem with the start-up and shutdown of the current controller. For example, it may take 0.1milliseconds to 1 millisecond to turn on and off the current. At highPWM operation frequencies, many other negative effects that the dimmableLED lighting system may encounter are the non-uniform dimming control,unpredictable control, and non-linear behavior. In applications withmultiple LED colors, the balance between a slow and fast PWM operationfrequency is important to the resulting color, and these issues severelylimit the ability to provide a desired resulting color.

There are also other modes of dimming that modify the intensity in waysother than by PWM operation that have desirable characteristics. One ofthe ways includes the use of delta-sigma modulation. However, the use ofdelta-sigma modulation would be impractical with the slow behavior ofthe controller. Thus, a control system that can operate linearly acrosswide dimming frequency ranges while maintaining high efficiency isdesired and needed.

SUMMARY OF THE INVENTION

In one embodiment of the present invention, a light emitting diode (LED)lighting system includes an LED controller to control a current in anLED system. The LED controller includes components to calculate, basedon the current and an active time period of an LED current time period,an actual charge amount delivered to the LED system and utilize theactual charge amount to modify and provide a desired target chargeamount to be delivered during a future active time period of the LEDcurrent time period. The LED system can also have components to comparethe actual charge amount to a desired charge amount for the active timeperiod and compensate for a difference between the actual charge amountand the desired charge amount during the future active time period.

In another embodiment of the present invention, a method of controllinga current in an LED system of an LED lighting system is disclosed. Themethod includes calculating, based on the current and an active timeperiod of an LED current time period, an actual charge amount deliveredto the LED system and utilizing the actual charge amount to modify andprovide a desired target charge amount to be delivered during a futureactive time period of the LED current time period. The method canfurther include comparing the actual charge amount to a desired targetcharge amount for the active time period and compensating for adifference between the actual charge amount and the desired chargeamount during the future active time period.

In a further embodiment of the present invention, a current controllerfor controlling current to an LED system for an LED lighting system isdisclosed. The current controller includes components for receiving adimming level signal from a dimming controller and for controlling andproviding, based on the dimming level signal, an amount of drive currentfor driving the LED system. The components are at least part of an LEDcontroller to calculate, based on the current and an active time periodof the LED current time period, an actual charge amount delivered to theLED system and to utilize the actual charge amount to modify and providea desired target charge amount to be delivered during a future activetime period of the LED current time period.

In a still further embodiment, a method for controlling current to anLED system for an LED lighting system is disclosed. The method includesreceiving a dimming level signal from a dimming controller andcontrolling and providing, based on the dimming level signal, an amountof drive current for driving the LED system. The receiving andcontrolling and providing steps are at least part of an LED controllermethod to calculate, based on the current and an active time period ofthe LED current time period, an actual charge amount delivered to theLED system and to utilize the actual charge amount to modify and providea desired target charge amount to be delivered during a future activetime period of the LED current time period.

In yet another embodiment, a delta-sigma modulator dimming controllerfor controlling a dimming level of an LED system for an LED lightingsystem is disclosed. The delta-sigma modulator dimming controllerincludes components for receiving a dimming control signal and driving adimming level signal to a current controller for providing a current fordriving the LED system. The components are at least part of an LEDcontroller to calculate, based on the current and an active time periodof an LED current time period, an actual charge amount delivered to theLED system and to utilize the actual charge amount to modify and providea desired target charge amount to be delivered during a future activetime period of the LED current time period.

In still yet another embodiment, a method for controlling a dimminglevel of an LED system for an LED lighting system utilizing a deltasigma modulator is disclosed. The method includes receiving a dimmingcontrol signal and driving a dimming level signal to a currentcontroller for providing a current for driving the LED system. Thereceiving and driving steps are at least part of an LED controllermethod to calculate, based on the current and an active time period ofan LED current time period, an actual charge amount delivered to the LEDsystem and to utilize the actual charge amount to modify and provide adesired target charge amount to be delivered during a future active timeperiod of the LED current time period.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerousobjects, features and advantages made apparent to those skilled in theart by referencing the accompanying drawings. The use of the samereference number throughout the several figures designates a like orsimilar element.

FIG. 1 (labeled prior art) depicts an LED lighting system.

FIG. 2 (labeled prior art) depicts a graphical representation of LEDcurrent in the LED lighting system of FIG. 1 for various dimming levels.

FIG. 3 (labeled prior art) depicts a graphical relationship between anenable signal, output voltage, and LED current plotted against dimmingvoltage values for a prior art LED switch mode controller.

FIG. 4 (labeled prior art) depicts another graphical relationshipbetween an enable signal, output voltage, and LED current plottedagainst dimming voltage values for a prior art LED switch modecontroller.

FIG. 5 (labeled prior art) depicts a graphical relationship between anenable signal, output voltage, and LED current plotted against time foranother prior art LED switch mode controller showing the ramp-up of theLED current.

FIG. 6 (labeled prior art) depicts another graphical relationshipbetween an enable signal, output voltage, and LED current plottedagainst time for the prior art LED switch mode controller which is theramp-down of the LED current shown in FIG. 5.

FIG. 7 depicts an LED lighting system having accurate current control inaccordance with the present invention.

FIG. 8 depicts a timing diagram for the current controller of thecontrolled LED lighting system which implements the principles of thepresent invention.

FIG. 9 depicts time plots showing a current active period of the LEDcurrent in which a Pulse Width Modulation (PWM) control signal controlsthe current levels of the LED current over the active time period of theLED current time period.

FIG. 10 depicts a time plot of LED current showing an exemplary train ofactive and inactive periods for the LED current controlled by a pulsewidth modulated dimming controller.

FIG. 11 depicts a time plot of LED current showing an exemplary train ofactive and inactive periods for the LED current controlled by a deltasigma modulated dimming controller.

DETAILED DESCRIPTION

A light emitting diode (LED) lighting system includes an LED controllerto accurately control a current in an LED system. The LED controllerincludes components to calculate, based on the current and an activetime period of an LED current time period, an actual charge amountdelivered to the LED system wherein the LED current time period is dutycycle modulated at a rate of greater than fifty (50) Hz and to utilizethe actual charge amount to modify and provide a desired target chargeamount to be delivered during a future active time period of the LEDcurrent time period. The LED system further has components to calculatefor an active time period of the LED current time period an actualcharge amount delivered to the LED system and also has components tocompare the actual charge amount to a desired charge amount for theactive time period of the LED current time period and compensate for adifference between the actual charge amount and the desired chargeamount during the future active time period. By being able to accuratelycontrol the desired charge amount, the average LED current is bettercontrolled, and thus, the light intensity of the LED(s) is moreeffectively controlled.

The accurate control and charge compensation of the LED current in thismanner and in accordance with the principles of the present inventionallows the LED lighting control system to operate linearly across widedimming ranges while maintaining high efficiency. By accuratelycontrolling and charge compensating the LED current, flicker caused by aslow PWM operation frequency (e.g., below 200 Hz) for pulsing the enableinput signal can be avoided. Additionally, start-up and shut downproblems caused by an overly fast PWM operation frequency for pulsingthe enable input signal are also avoided by accurately calculatingcharge compensation for the LED current. Other negative effects causedby an overly fast PWM operation frequency, such as non-uniform dimmingcontrol, unpredictable control, and non-linear behavior, are alsoeliminated because of the accurate control and charge compensation ofthe LED current. By being able to accurately balance a slow and fast PWMoperation frequency, the ability to provide a desired resulting LEDcolor is no longer limited.

FIG. 7 depicts an LED lighting system 700 that includes a currentcontrol system 702 to control the LED current i_(LED). The LED lightingsystem 700 also includes a dimming strategy module 704 to vary andmodulate an active value of LED current i_(LED) in response to varyingdimming levels and in accordance with a dimming strategy describedsubsequently in more detail. In at least one embodiment, the LEDlighting system 700 also includes the voltage source 104, dimmer 106,rectifier 108, and switching power supply 110, which operate aspreviously described.

The current control system 702 (shown in a dotted-line border) includesan LED controller 706 to generate a duty cycle modulated gate controlsignal C_(G1) to control conductivity of FET Q1 and, thus, control LEDcurrent i_(LED). LED controller 706 includes a dimming controller 707and current controller 709. Dimming controller 707 drives currentcontroller 709. Dimming controller 707 can be a pulse width modulation(PWM) dimming controller or a delta-sigma modulated dimming controller.Control signal C_(G1) charges and discharges a gate of FET Q₁. A logical1 value (e.g., a first state) of control signal C_(G1) causes FET Q₁ toconduct and draw LED current i_(LED) through an LED system thatcomprises a number of LEDs 102 and also through an inductor L1. Alogical 0 value of control signal C_(G1) causes FET Q1 to benon-conductive (e.g., a second state). FET Q₁ represents one embodimentof a switch and can be replaced by any type of switch.

In at least one embodiment, the LED lighting system 700 dims the LEDsystem (e.g., the LEDs 102) in conformity with a dimming level inputgenerated by a dimmer such as phase cut dimmer 106. The number of LEDs102 is a matter of choice. LEDs 102 can be replaced by a single LED. TheLED lighting system 700 can receive dimmer signals indicating dimminglevels from LEDs 102 from any type of dimmer. For example, dimmer 106can be omitted, and LED lighting system 700 can include a dimmer, suchas digital dimmer 708 or a dimmer 106 having a direct current (DC)dimming control voltage (not shown). In at least one embodiment, thedigital dimmer 708 is a digital addressable lighting interface (DALI)compatible dimmer. Digital dimmer 708 is depicted with “dashed” linesbecause generally LED lighting system 700 includes one dimmer or anotherdimmer but not two dimmers. Thus, in at least one embodiment, digitaldimmer 708 is a substitute for dimmer 106 and phase delay detector 710.The dimmers, such as dimmer 106 and digital dimmer 708, receive inputs,either manually or automatically, that set the dimming level values tobe output by the dimmers.

In at least one embodiment, the LED controller 706 responds to a dimminglevel input and generates the control signal C_(G1) in accordance with adimming strategy that, in at least one embodiment, includes two modes ofoperation. In an active value varying mode of operation, the LEDcontroller 706 varies an active value of the LED current i_(LED) inconformity with the dimming level for a first set of dimming levels. Inan active value, duty cycle modulation mode of operation, the LEDcontroller 706 modulates a duty cycle of an active value of the LEDcurrent it i_(LED) in conformity with the dimming level for a second setof dimming levels.

To determine which of the two modes of operation is to be used ingenerating the LED current i_(LED), LED lighting system 700 firstdetects a dimming level for LEDs 102. When LED lighting system 700includes dimmer 106, the LED lighting system 700 also includes a phasedelay detector 710 to detect phase delays in the phase modulated signalV_(Φ). The phase delay detector 710 generates a phase delay signal Φ,and the phase delays represented by the digital phase delay signal Φrepresent dimming levels. Melanson III describes an exemplary embodimentof phase delay detector 710.

In at least one embodiment, the LED lighting system 700 also includes anoptional mapping system and filter 711 to map the dimming levelsindicated by the phase delay signal Φ to predetermined digital values ofdimming signal D_(V). Melanson IV describes an exemplary mapping systemand filter 711 that maps values of dimming signal D_(V) to perceivedlight levels. The LED lighting system 700 receives the dimming signalD_(V) as a dimming level input. In at least one embodiment, LED lightingsystem 700 omits the mapping system and filter 711, and the dimmingstrategy module 704 receives the phase delay signal Φ as a direct,digital dimmer signal input having values indicating dimming levels.

FIG. 8 depicts an exemplary timing diagram for the dimming controller707 and/or the current controller 709 of LED controller 706 withincontrolled LED lighting system 700. Dimming controller 707 and currentcontroller 709 can each be implemented as a time-based controller thatcontrols FET Q₁ such that the output voltage V_(out) of controlled LEDlighting system 700 has a desired average value. Because the controlapplied to FET Q₁ by time-based dimming controller 707 or time-basedcurrent controller 709 always causes controlled LED lighting system 700to integrate up or down (e.g., integration response), time-based dimmingcontroller 707 or time-based current controller 709 is said to applybang-bang control.

As indicated by its name, time-based dimming controller or time-basedcurrent controller 709 implements a time-based control methodology,rather than one of the conventional magnitude-based controlmethodologies. Time-based dimming controller 707 or time-based currentcontroller 709 receives a compared voltage V_(COMP) which is acomparison of sensed signal LEDi_(sense) indicative of a current orvoltage (e.g., sense current i_(LEDsense)) in controlled LED lightingsystem 700 and a target or reference signal i_(target)(t), such as ananalog or digital current or an analog or digital voltage provided fromdimming strategy module 704. In the depicted timing diagram, sensedsignal LEDi_(sense) is, for example, the current i_(LEDsense) sensed atthe drain of FET Q1 going through resistor R_(sense), as shown in FIG.7, and the target current i_(target)(t) is a target current i_(TARGET)provided from dimming strategy module 704. Of course, in alternativeembodiments, sensed signal LEDi_(sense) and the target signali_(target)(t) may both be voltages.

While a control signal C_(G1) supplied to the LED system (LEDs 102) isin a first state (e.g., such as an on-state), a polarity change in acomparison of the sensed signal LEDi_(sense) and the target/referencesignal i_(target)(t) is detected at a first time. Based on the firsttime, a second time is determined at which to change a state of thecontrol signal C_(G1) supplied to the LED system (LEDs 102). At thedetermined second time, the state of the control signal C_(G1) suppliedto the LED system (LEDs 102) is changed from the first state to a secondstate (e.g., such as an off-state).

In FIG. 8, sensed signal LEDi_(sense), which is either rising or fallingat all times (e.g., polarity change), has repeating cycles of period Peach comprising an interval T1 in which sensed signal LEDi_(sense) isrising and an interval T2 in which sensed signal LEDi_(sense) isfalling. Each interval T1 in turn comprises an interval A (e.g., A(0),A(1), etc.) during which sensed signal LEDi_(sense) rises from a cycleinitial value (e.g., one state) to the target signal i_(target)(t) and asubsequent interval B during which sensed signal LEDi_(sense) rises fromthe target signal i_(target)(t) to a cycle maximum value (e.g., anotherstate). Sensed signal LEDi_(sense) falls from the cycle maximum value tothe initial value of the next cycle during interval T2. For clarity,intervals A and B are identified with ascending numerical cycle indices(A(0), A(1), etc. and B(0), B(1), etc.).

In accordance with the present invention, time-based dimming controller707 or time-based current controller 709 can control FET Q₁ to implementany of a number of time-based control methodologies. For example,time-based dimming controller 707 or time-based current controller 709can implement constant period control so that period P is constant (andintervals T1 and T2 vary between cycles), or constant on-time control sothat interval T1 is constant (and period P and interval T2 vary betweencycles), or constant off-time control so that interval T2 is constant(and period P and interval T1 vary between cycles). A desiredmethodology may be selected, for example, to reduce electromagneticinterference (EMI) with surrounding circuitry.

The simplest control methodology, which also enables an immediate lockto the target signal i_(target)(t), is a constant on-time or constantoff-time approach in which one of intervals T1 or T2 is of constantduration and the other interval (and period P) varies in duration. In aconstant off-time control methodology, time-based dimming controller 707or time-based current controller 709 controls FET Q₁ such that theinterval A of interval T1 during which the sensed signal LEDi_(sense) isless than the target signal i_(target)(t) and the interval B of intervalT1 during which the sensed signal LEDi_(sense) is greater than thetarget signal i_(target)(t) are equal. According to this constantoff-time control methodology, the duration of interval B for each cycleis determined in accordance with the following equation 1:B(N)=[B(N−1)+A(N)]/2,  (Equation 1)where N is the cycle index. Thus, for example, utilizing Equation 1,time interval B(1) is equal to the average of time intervals B(0) andA(1). Interval T2 is, of course, fixed in duration.

The constant on-time control methodology employs the same equation asthe constant off-time approach, except that in the constant on-timeapproach, interval T1 is of constant duration, interval A is the portionof interval T2 in which the sensed signal LEDi_(sense) exceeds thetarget signal i_(target)(t), and interval B is the portion of intervalT2 in which the sensed signal LEDi_(sense) is less than the targetsignal i_(target)(t). Time-based current controller 709 again controlsFET Q₁ such that intervals A and B are of equal duration.

With reference now to FIG. 9, a current-lime plot 900 and a controlsignal time plot 902 are shown. An LED current time period for an LEDcurrent i_(LED) includes both active time periods and inactive timeperiods. An exemplary active time period of an LED current time periodutilized by controlled lighting system 700 for dimming control is shownin current-time plot 900. A Pulse Width Modulation (PWM) switchingcontrol signal C_(G1) is shown in control signal time plot 902 plottedover the same active period (e.g., from 0 to 120 microseconds). The PWMswitching control signal C_(G1) controls the current levels of the LEDcurrent i_(LED) over the active time period for controlling the dimminglevels of the LEDs 102. The active time period is generally defined asan LED current pulse (e.g., LED current pulse 901), that is, from whenthe current level of the LED current i_(LED) ramps up to and fluctuatesat an average high current value i_(high) (e.g., 0.45 Amp) and throughand until the time when the current level of the LED current i_(LED)ramps down to a low current value i_(low) (e.g., 0 Volts).

As shown in FIG. 9, the PWM switching frequency f_(SW) for the PWMcontrol signal C_(G1) is different than the PWM dimming frequency ratef_(DIM) for the LED current i_(LED) for controlling dimming levels ofLEDs 102 over time. Thus, for the operations of LED lighting system 700,two different PWM operating frequencies are respectively being utilizedfor the control signal C_(G1) and the dimming control (e.g., control ofthe levels of LED current i_(LED)). Exemplary operating frequencies forPWM dimming frequency rate f_(DIM) widely ranges from 100 Hz to 20 kHz.The PWM dimming frequency rate f_(DIM) is provided by duty cyclemodulating the LED current time period at a rate of greater than fifty(50) Hz. Exemplary operating frequencies for PWM switching frequencyf_(SW) range from 50 kHz to 250 kHz.

In FIG. 9, while control signal C_(G1) turns on and has a high value(e.g., 1), LED current i_(LED) ramps up during 0 microsec. to 12.5microsec. in accordance to ramp-up slope R_(UP1) and charges up to 0.45Amp. When control signal C_(G1) turns off and has a low value (e.g., 0),LED i_(LED) starts decreasing from 0.45 Amp through 0.4 Amp and reaches0.35 Amp at which point control signal C_(G1) turns back on and has ahigh value (e.g., 1). The current level of LED current i_(LED) continuesto fluctuate in this manner (e.g., between 0.45 Amp and 0.35 Amp) inaccordance with the pulses (e.g., turning on and off FET Q₁ of LEDlighting system 700) of control signal C_(G1). The fluctuations of theLED current level span from 12.5 microsec. through 92.5 microsec.

From 92.5 microsec. to 120 microsec., the current level of LED currenti_(LED) ramps down in accordance with ramp-down slope R_(DN1) from 0.4Amp to 0 Amp since control signal C_(G1) is turned off and stays at 0.The actual charge amount Q_(Actual) for LED current pulse 901 iscalculated as follows:Q _(Actual) =Q1+Q2−Q3  Equation 2

The following charge amounts are determined by the following areacalculations:Q1=½*(X1*Y)=½*((12.5−0)*0.4)=2.5 μCoulombs  Equation 3Q2=Z1*Y=(92.5−12.5)*0.4=32 μCoulombs  Equation 4Q3=½*(X2*Y)=½*((120−92.5)*0.4)−5.5 μCoulombs  Equation 5

Thus, the total actual charge amount Q_(Actual) for LED current pulse901 is:Q _(Actual)=2.5+32+5.5=40 μCoulombs  Equation 6

However, due to discrete limitations (e.g., discrete time/steps) ofcharge quantization, the total actual charge amount Q_(Actual) for anactive time period (e.g., LED current pulse 901) may differ from what atotal desired charge amount Q_(Desire) is. Thus, the total desiredcharge amount Q_(Desire) that is desired to be delivered to LEDs 102 iscalculated as follows:Q _(Desire) =Q1+Q2+Q3+/−Q _(error)  Equation 7The quantization error charge amount Q_(error) may be a deficient chargeamount or an excess charge amount depending on what the total desiredcharge amount Q_(Desire) is relative to what the total actual chargeamount Q_(Actual) that can actually be delivered. If the quantizationerror charge amount Q_(error) is a deficient charge amount, then thequantization error charge amount Q_(error) is compensated by adding theequivalent charge amount in during a next or future time period (e.g.,future LED current pulse) of LED current time period. For example, ifthe actual charge amount is 40 μCoulombs, but 41 μCoulombs is thedesired charge amount Q_(Desire) and cannot be achieved due to chargequantization limitations, then the quantization error charge amountQ_(error) is a deficiency of 1 μCoulomb (e.g.,Q_(error)=Q_(Actual)−Q_(Desire)=40 μCoulombs−41 μCoulombs=−1 μCoulomb).In this case, 1 μCoulomb is added in during a next of future time periodto compensate the actual charge amount Q_(Actual) for the desired chargeamount Q_(Desire). On the other hand, if the quantization error chargeamount Q_(error) is an excess charge amount, then the quantization errorcharge amount Q_(error) is compensated by subtracting an equivalentcharge amount from a next or future time period (e.g., future LEDcurrent pulse) of LED current time period. For example, if the actualcharge amount is 40 μCoulombs, but 39 μCoulombs is the desired chargeamount Q_(Desire) and cannot be achieved due to charge quantizationlimitations, then the quantization error charge amount Q_(error) is anexcess amount of 1 μCoulomb (e.g., Q_(error)=Q_(Actual)−Q_(Desire)=40μCoulombs−39 μCoulombs=+1 μCoulomb). In this case, 1 μCoulomb issubtracted from a next of future time period to compensate the actualcharge amount Q_(Actual) for the desired charge amount Q_(Desire).

The process for modifying charge amounts delivered at a future time(e.g., modifying the charge amounts for future LED current pulses) asdiscussed for FIG. 9 can be appropriately repeated for subsequent LEDcurrent pulses. In the same manner for subsequent LED current pulses,charge amounts would be respectively compensated by adding to orsubtracting from charge amounts of future LED current pulses dependingupon whether the error charge amount is respectively a deficient orexcess charge amount relative to the desired charge amount.

The dimming controller 707 can be a pulse width modulation (PWM) dimmingcontroller or can be a delta-sigma dimming controller. Referring now toFIG. 10, a time plot 1000 of LED current shows an exemplary train of LEDcurrent pulses (e.g., current pulses 1002, 1004, 1006, and 108) that areindicative of active time periods of an LED current i_(LED). The timeplot 1000 also shows the inactive periods generally equally spaced apartbetween the LED current pulses since the dimming controller 707 is apulse width modulated dimming controller. Error charge amounts thatoccur during an earlier active time period is compensated during asubsequent or future active time period. In other words, an error chargeamount that occurs during current pulse 1002 is compensated duringcurrent pulse 1004, and an error charge amount occurring during currentpulse 1004 is compensated during current pulse 1006. An error chargeamount occurring during current pulse 1006 is compensated during currentpulse 1008.

Referring now to FIG. 11, a time plot 1100 of LED current shows anexemplary train of LED current pulses (e.g., current pulses 1102, 1104,1106, 1108, 1110, 1112, 1114) that are indicative of active periods ofan LED current i_(LED). The time plot 1100 also shows the inactiveperiods non-uniformly spaced apart between the LED current pulses sincethe dimming controller 707 is a delta-sigma modulated dimmingcontroller. Again, error charge amounts that occur during an earlieractive time period is compensated during a subsequent or future activetime period. In other words, an error charge amount that occurs duringcurrent pulse 1102 is compensated during current pulse 1104, and anerror charge amount occurring during current pulse 1104 is compensatedduring current pulse 1106 and so on and so forth. As shown in FIG. 11,the pulses 1102, 1104, 1106, 1108, 1110, 1112, 1114 may be the same ordifferent in duration and may or may not be generally uniformly spacedpart.

The use of a delta-sigma modulated dimming controller 707 instead of aPWM dimming controller 707 for controlling the LED current i_(LED) inFIG. 7 provides the characteristic of broadening the signal spectrum,which minimizes the potential for audible tones. A simple second ordermodulator, with reasonable dither level, may be implemented for thedelta-sigma modulator, and such an implementation is generallysufficient and relatively inexpensive to implement. For example, theswitching frequency f_(SW) may be 200 kHz while the delta-sigma dimmingfrequency rate f_(DIM) may be 20 kHz. In this case, a minimal chance forany audio noise production exists. However, a switching current controlwith rapid response, as it is turned on and off at a fast rate, isneeded. Thus, a time-based dimming controller 707 and a time-basedcurrent controller 709 as discussed earlier for FIG. 7 provide such afast switching response.

Thus, the actual charge amount delivered to the LEDs 102 is calculatedand accumulated. The charge accumulation is compared to the desiredcharge amount. Modification and compensation of the total charge amountdelivered to the LEDs 102 can be continuously and constantly performed,which can at least compensate for error charge amounts. Regardless ofthe characteristics of the start-up and start-down of the LED controller706, the LED lighting system 700 will properly compensate and allows fora much faster PWM switching rate f_(SW). Such a feature allows forsmooth dimming of LEDs 102 by LED lighting system 700.

Exemplary pseudo-code for PWM operation of dimming control 707 isprovided as follows:

Dim level D, 0-1

Qint charge accumulation, initialized to 0

PWM period PP

Full-scale current Itarget

Current control sample period PCC

Instantaneous LED current LEDI

At PP rate, Qint=Qint+D*Itarget

At PCC,

Qint=Qint−PCC*LEDI

If Qint>0, turn on LED controller

If Qint<=, turn off LED controller

Although the present invention has been described in detail, it shouldbe understood that various changes, substitutions and alterations can bemade hereto without departing from the spirit and scope of the inventionas defined by the appended claims.

What is claimed is:
 1. A light emitting diode (LED) lighting systemcomprising: a switching power supply for converting a phase modulatedsignal generated from a dimmer into an output voltage, wherein thedimmer is a phase cut dimmer and operates at a repetitive phase cutrate; an LED controller coupled to the switching power supply; and lightemitting diodes (LEDs) coupled to the LED controller; and wherein theLED controller controls a current of the LEDs and includes componentsto: calculate, based on the current of the switching power supply, anaverage current amount delivered to the LEDs in a time period, whereinthe time period is based on the repetitive phase cut rate; and utilizean actual charge based on the current to modify a future switchingtiming of the switching power supply.
 2. The LED lighting system ofclaim 1 wherein the LED controller controls dimming of the LEDs.
 3. TheLED lighting system of claim 2 wherein the LED controller controlsdimming of the LED system over a wide dimming frequency range.
 4. TheLED lighting system of claim 1 further comprising: components to utilizethe actual charge based on the current to modify the future switchingtiming of the switching power supply.
 5. The LED lighting system ofclaim 4 wherein the components to utilize the actual charge furthercomprises: components to compare the actual charge amount to a desiredcharge amount for an active time period and compensate for a differencebetween the actual charge amount and the desired charge amount during afuture active time period.
 6. The LED lighting system of claim 5 whereinthe desired target charge is based on phase delays in a rectified inputvoltage.
 7. The LED lighting system of claim 5 wherein the active timeperiod and the future active time period are based on an averagingperiod that is based on the repetitive phase cut rate.
 8. The LEDlighting system of claim 7 wherein the LED controller controls dimmingof the LEDs.
 9. The LED lighting system of claim 8 wherein the LEDcontroller comprises a dimming controller and a current controllerwherein the dimming controller drives the current controller.
 10. TheLED lighting system of claim 9 wherein the dimming controller is drivenby the repetitive phase cut rate and the current controller is driven bya switching frequency rate and wherein the repetitive phase cut rate isdifferent than the switching frequency rate.
 11. A method forcontrolling a current in light emitting diodes (LEDs) of an LED lightingsystem, comprising: converting, by a switching power supply, a phasemodulated signal generated from a dimmer into an output voltage, whereinthe dimmer is a phase cut dimmer and operates at a repetitive phase cutrate; controlling, by the LED controller coupled to the switching powersupply and the light emitting diodes, a current of the LEDs;calculating, by the LED controller, based on the current of theswitching power supply, an average current amount delivered to the LEDsin a time period, wherein the time period is based on the repetitivephase cut rate; and utilizing, by the LED controller, the actual chargebased on the current to modify a future switching timing of theswitching power supply.
 12. The method of claim 11 wherein controlling,by the LED controller coupled to the switching power supply and thelight emitting diodes, a current of the LEDs further comprises:controlling, by the LED controller, dimming of the LEDs.
 13. The methodof claim 12 wherein controlling, by the LED controller, dimming of theLEDs further comprises: controlling, by the LED controller, dimming ofthe LEDs over a wide dimming frequency range.
 14. The method of claim 11further comprising: utilizing, by components, the actual charge based onthe current to modify the future switching timing of the switching powersupply.
 15. The method of claim 14 further comprises: comparing, bycomponents, the actual charge amount to a desired charge amount for anactive time period; and compensating, by the components, for adifference between the actual charge amount and the desired chargeamount during a future active time period.
 16. The method of claim 15wherein the desired target charge is based on phase delays in arectified input voltage.
 17. The method of claim 15 wherein the activetime period and the future active time period are based on an averagingperiod that is based on the repetitive phase cut rate.
 18. The method ofclaim 11 wherein utilizing, by the LED controller, the actual charge tomodify and provide a desired target charge amount to be deliveredfurther comprises: utilizing the actual charge to modify and provide thedesired target charge amount to be delivered during the future activetime period of the LED current time period, wherein the future activetime period is based on the repetitive phase cut rate further comprises;comparing the actual charge amount to a desired charge amount for theactive time period; and compensating for a difference between the actualcharge amount and the desired charge amount during the future activetime period.
 19. The method of claim 18 wherein controlling, by the LEDcontroller coupled to the switching power supply and the light emittingdiodes, a current, further comprises: controlling, by the LEDcontroller, dimming of the LEDs.
 20. The method of claim 19 wherein theLED controller comprises a dimming controller and a current controllerand further comprising: driving, by the dimming controller, the currentcontroller.
 21. The method of claim 20 wherein driving, by the dimmingcontroller, the current controller further comprises: driving thedimming controller at the repetitive phase cut rate; and driving thecurrent controller at a switching frequency rate that is different thanthe repetitive phase cut rate.